1. Field of the Invention
The present invention relates to an image-processing apparatus and an image-processing method.
2. Description of the Related Art
Hitherto, multiplexing systems wherein multiplexing of additional information is united with redundancy of pseudo-halftone processing have been proposed.
According to a multiplexing system disclosed in Japanese Patent Laid-Open No. 63-214067 (hereinafter referred to as “the first multiplexing system”), data is mixed with an image signal by selecting any one of dither matrices expressing the same gradation at the time where binarization is performed according to an organized dithering method. Further, according to a multiplexing system disclosed in Japanese Patent Laid-Open No. 01-292965 (hereinafter referred to as “the second multiplexing system”), a color dither-pattern method is used so that additional information is multiplexed based on an array used for the color dither-pattern method.
Further, multiplexing systems from which high robustness can be expected have been proposed. According to a multiplexing system disclosed in Japanese Patent Laid-Open No. 2001-148778 (hereinafter referred to as “the third multiplexing system”), a quantization threshold value used for an error-diffusion method is subjected to amplitude modulation and the periodicity of the modulation is changed into predetermined blocks based on code of additional information so that additional information is multiplexed on data on a printed image. Decoding of the multiplexed additional information is specified by relatively comparing orthogonal-transformation coefficients obtained for each of predetermined bands to each other based on the premise that power of a predetermined frequency band increases for each of predetermined blocks. Contrasted with the first and second multiplexing systems, the precision of decoding performed in the third multiplexing system does not significantly vary due to phase information indicating the precise positions of fine dots, distances between dots, etc. According to the third multiplexing system, the additional information is multiplexed in the plural bands corresponding to frequencies lower than quantization frequencies used for the error-diffusion method. Therefore, the third multiplexing system is resistant to some deflection of dots, the smear of ink droplets, etc., and high robustness can be expected.
According to the third multiplexing system, however, the band where the additional information is multiplexed is lower than that of the quantization frequencies. Therefore, visual deterioration of the quality of an image shown on paper becomes more conspicuous than in the case where the first and second multiplexing systems are used. According to the third multiplexing system, the image-quality deterioration can be reduced by decreasing the amount of code for multiplexing for each of predetermined blocks. In that case, however, the amount of information that can be multiplexed is limited.
As digital data becomes widely available, the desire to increase the amount of additional information that can be multiplexed on paper grows. However, no appropriate large-capacity multiplexing assuring image quality and robustness has been achieved through any of known multiplexing systems.